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Week Number Calculator

Find the ISO 8601 or US week number of any date, or the first and last day of any numbered week in a year.

Level: Grades 4–5

Printable worksheet

Week Number Calculator

Inputs
What do you want to find?
Week numbering

Choose a date.

Result

Calculating…

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

What this calculator does

Find which numbered week of the year any date falls in, or which days a numbered week covers. Many offices, factories, schools and software teams label time by week number instead of by date. You'll hear "the release is planned for week 42" or "invoices for W07". The trouble is, there's more than one way to number weeks. The international standard, ISO 8601, starts weeks on Monday. It also lets a few days at the turn of the year belong to the year next door. Many US calendars and spreadsheets start weeks on Sunday instead, and begin week 1 on January 1.

This calculator handles both. Give it a date to get the week number, the span of that week, and the number of weeks in the year. Or give it a year and a week number to see exactly which days that week covers. The working shows each rule applied to your dates.

Where it's used

Why this math matters outside the classroom:

  • Project schedules. Teams plan a release for ISO week 42 of 2026, which runs from Monday, October 12 to Sunday, October 18.
  • Payroll and timesheets. Payroll clerks label each pay period with its week number so timesheets from different months line up.
  • Shipping calendars. Factories quote delivery as a calendar week, such as week 10 of 2026, Monday March 2 to Sunday March 8.
  • School planning. Teachers number the weeks of a term so lesson plans can be reused from one year to the next.
  • Remainder practice. Students divide 287 by 7 to get 41 whole weeks and add 1, so October 15, 2026 is in week 42.

How to use it

  1. Choose whether you want the week number of a date, or the dates of a week number.
  2. Choose ISO 8601 or US numbering. ISO starts on Monday and is used across Europe and in most software standards. US starts on Sunday, and week 1 contains January 1.
  3. Enter the date, or the year and the week number.
  4. Read the week number or the date span. For ISO, also check which year the week belongs to. In early January and late December, it can differ from the date's own year.

What the inputs mean

Week numbering
ISO 8601 week 1 is the week with the year's first Thursday in it. That's the same as the week containing January 4. US numbering puts January 1 in week 1, whatever day it falls on, so its first and last weeks can be short.
Week number
ISO years have 52 or 53 weeks. US-numbered years have 53 weeks, or 54 when a leap year starts on a Saturday.

Formula and method

ISO week = ⌊(day of the year of that week's Thursday − 1) ÷ 7⌋ + 1
ISO week 1 starts on the Monday on or before January 4
US week = ⌊(day of the year + weekday of January 1 − 1) ÷ 7⌋ + 1   (Sunday = 0 … Saturday = 6)
start of week w = start of week 1 + 7 × (w − 1) days

In the formulas, the ⌊ ⌋ brackets mean "round down to a whole number", and w is the week number. The "day of the year" counts the days from the start of January. The weekday the year starts on is numbered from Sunday up to Saturday, as shown.

Under ISO 8601, every week is a full Monday-to-Sunday block. Each week is filed under the year that contains its Thursday, the middle day. Find that Thursday, count the whole weeks before it in its year, and you have the week number. January 1 can fall late in a week. So up to three days at the start of January may belong to week 52 or 53 of the year before. And up to three days at the end of December may be week 1 of the next year.

A year has 53 ISO weeks when it starts on a Thursday, or when it's a leap year starting on a Wednesday. Otherwise it has 52.

US numbering is simpler to explain. Week 1 is whatever Sunday-to-Saturday week contains January 1, and the week number goes up every Sunday after that. Adding the weekday of January 1 to the day of the year lines the count up with those Sundays. Every date stays in its own calendar year, so December 31 can be in week 53 or 54.

To go from a week number to dates, find the first day of week 1 and step forward whole weeks.

Worked example

Which week is Sunday, October 18, 2026 in?

  1. ISO: the week runs Monday, October 12 to Sunday, October 18. Its Thursday, October 15, is day 288 of 2026.
  2. ISO week = ⌊(288 − 1) ÷ 7⌋ + 1 = ⌊41⌋ + 1 = 42.
  3. US: January 1, 2026 is a Thursday (weekday 4), and October 18 is day 291.
  4. US week = ⌊(291 + 4 − 1) ÷ 7⌋ + 1 = 42 + 1 = 43, the week of Sunday, October 18 to Saturday, October 24.

Result: October 18, 2026 is in ISO week 42 (written 2026-W42-7) and in US week 43.

Understanding the result

The two systems often differ by one, as in the example. That's because they disagree on the first day of the week and on where week 1 begins.

The ISO label 2026-W42-7 means ISO year 2026, week 42, day 7 (Sunday). Day 1 is Monday.

When someone gives you only a week number, ask which system they use. European and international schedules nearly always mean ISO weeks.

Assumptions and limitations

  • Dates are on the Gregorian calendar (the everyday calendar) and are treated as calendar days, so time zones don't matter.
  • US numbering follows the common spreadsheet rule: weeks start on Sunday, and week 1 contains January 1.
  • Other systems exist and aren't covered. Examples are Middle Eastern calendars with weeks that start on Saturday, and fiscal years that begin in another month.
  • Under US numbering, the first and last weeks of a year are partial. The calculator shows their full Sunday-to-Saturday span.

Frequently asked questions

What week number is it today?

Leave the date on today and read the result. Make sure the week-numbering choice matches the calendar you're comparing with. Use ISO for most European, international and software calendars, and US for many US wall calendars.

Why does January 1 sometimes fall in week 52 or 53?

Under ISO 8601, a week belongs to the year of its Thursday. When January 1 is a Friday, Saturday or Sunday, its week's Thursday is in December. So those days count as the last week of the year before.

Which years have 53 ISO weeks?

Years that start on a Thursday, and leap years that start on a Wednesday. Recent and upcoming examples are 2015, 2020, 2026 and 2032.

Is the ISO week the same as the week number in my spreadsheet?

Not always. Spreadsheet week-number functions often use the US system by default. Most offer an ISO option as well, which gives the same results as the ISO setting here.

How this calculator is checked

Method: ISO 8601 week-date rules (week 1 contains the first Thursday); US week numbering with Sunday start and week 1 containing January 1. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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